U.S. patent number 6,937,780 [Application Number 10/220,897] was granted by the patent office on 2005-08-30 for multi-pass, arcuate bent waveguide, high power super luminescent diode.
This patent grant is currently assigned to Trumpf Photonics, Inc.. Invention is credited to Gerard A. Alphonse.
United States Patent |
6,937,780 |
Alphonse |
August 30, 2005 |
Multi-pass, arcuate bent waveguide, high power super luminescent
diode
Abstract
An optical device (300) including first and second facets (340,
350); an at least partially bent waveguide (320) formed on a
substrate and including a portion perpendicular to the first facet;
and a light amplification region (310) coupled to the bent
waveguide. The light amplification region includes an expanding
tapered portion and a contracting tapered portion which approaches
the second facet.
Inventors: |
Alphonse; Gerard A. (Princeton,
NJ) |
Assignee: |
Trumpf Photonics, Inc.
(Cranbury, NJ)
|
Family
ID: |
22679738 |
Appl.
No.: |
10/220,897 |
Filed: |
February 5, 2003 |
PCT
Filed: |
February 23, 2001 |
PCT No.: |
PCT/US01/06039 |
371(c)(1),(2),(4) Date: |
February 05, 2003 |
PCT
Pub. No.: |
WO01/63331 |
PCT
Pub. Date: |
August 30, 2001 |
Current U.S.
Class: |
385/14;
257/E33.054; 385/43 |
Current CPC
Class: |
G02B
6/1228 (20130101); G02B 6/125 (20130101); H01L
33/0045 (20130101); H01S 3/063 (20130101); G02B
2006/12107 (20130101); G02B 2006/12119 (20130101); G02B
2006/12123 (20130101); H01S 5/10 (20130101); H01S
5/1064 (20130101); H01S 5/14 (20130101) |
Current International
Class: |
G02B
6/125 (20060101); G02B 6/122 (20060101); H01L
33/00 (20060101); G02B 6/12 (20060101); G02B
006/12 () |
Field of
Search: |
;385/14,43,32,129-132
;359/326,328,344 ;257/98 ;372/1,6,7,45,97,98 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0318947 |
|
Jun 1989 |
|
EP |
|
WO 9747042 |
|
Dec 1997 |
|
WO |
|
WO 9966613 |
|
Dec 1999 |
|
WO |
|
Other References
Lin, C. P. et al., "Superluminescent Diodes With Bent Waveguide,"
IEEE Phototonics Technology Letters, Feb. 1996, vol. 8, No. 2, pp.
206-208. .
Semenov, A.T. et al., "Wide Spectrum Single Quantum Well
Superluminescent Diodes at 0.8.mu.m With Bent Optical Waveguide,"
Electronics Letters, May 1993, vol. 29, No. 10, pp. 854-855. .
Vogel Verlag K.G., "Optischer Verstaerker mit Niedriger
Superlumineszenz," Neus Aus Der Technik, Wurzburg, DE, No. 6, vol.
1, p. 2 (1981) and English translation..
|
Primary Examiner: Bovernick; Rodney
Assistant Examiner: Stahl; Mike
Attorney, Agent or Firm: Fish & Richardson P.C.
Parent Case Text
RELATED APPLICATION
This application claims priority of U.S. patent application Ser.
No. 60/185,133, entitled "DOUBLE-PASS HIGH POWER SUPERLUMINESCENT
DIODE (SLD) AND OPTICAL AMPLIFIER WITH MODE STABILIZATION", filed
Feb. 25, 2000, the entire disclosure of which is hereby
incorporated by reference herein.
Claims
What is claimed is:
1. An optical device comprising: a first facet; a second facet; an
at least partially arcuate waveguide formed on a substrate and
including a portion coextensive to said first facet, a portion that
is not perpendicular to the first or second facet, and a light
amplification region, said light amplification region including an
expanding tapered portion and a contracting tapered portion within
the portion of the waveguide that is not perpendicular to the first
or second facet, wherein said contracting tapered portion
approaches said second facet.
2. The device of claim 1, wherein said contracting region
approaches said second facet of said optical device at an angle of
about six degrees to perpendicular.
3. The device of claim 2, wherein said at least partially arcuate
waveguide has a width of about three microns.
4. The device of claim 3, wherein said at least partially arcuate
waveguide and amplification region are formed on a substrate
including said first and second facets and an index of refraction
difference between said at least partially arcuate waveguide and
amplification region and said substrate .ltoreq. approximately
0.01.
5. The device of claim 1, wherein said waveguide is a ridge
waveguide.
6. The device of claim 1, wherein at least a portion of said at
least partially arcuate waveguide has a radius of curvature
sufficiently large to curtail radiation of a mode propagating
through said waveguide due to the curvature thereof.
7. The device of claim 6, wherein said radius of curvature is on
the order of 10 mm.
8. The device of claim 1, further comprising a fiber optic coupled
to said tapered portion of said waveguide.
9. The device of claim 8, wherein said fiber is doped so as to
perform upconversion of light passing through said second
facet.
10. The device of claim 9, further comprising a first plurality of
frequency selective reflectors coupled to said fiber.
11. The device of claim 10, further comprising a second plurality
of frequency selective reflectors, each of said second plurality of
reflectors coupled to one of said first plurality of
reflectors.
12. The device of claim 11, wherein each of said first plurality of
reflectors is highly reflective for at least one select frequency
and highly transmissive for at least one other frequency.
13. The device of claim 12, wherein at least one of said second
plurality of reflectors is highly reflective for at least one
select frequency and highly transmissive for at least one other
frequency.
14. The device of claim 13, wherein at least one other of said
second plurality of reflectors is only partially reflective for
said at least one select frequency that said at least one of said
second plurality of reflectors is highly reflective.
15. The device of claim 1 further comprising a crystal coupled to
said second facet for performing upconversion of light passing
through said second facet.
16. The device of claim 15, wherein said crystal is a periodically
poled lithium niobate crystal.
17. The device of claim 15, further comprising a first plurality of
frequency selective reflectors coupled to said crystal.
18. The device of claim 17, further comprising a second plurality
of frequency selective reflectors, each of said second plurality of
reflectors coupled to one of said first plurality of
reflectors.
19. The device of claim 18, wherein each of said first plurality of
reflectors is highly reflective for at least one select frequency
and highly transmissive for at least one other frequency.
20. The device of claim 19, wherein at least one of said second
plurality of reflectors is highly reflective for at least one
select frequency and highly transmissive for at least one other
frequency.
21. The device of claim 20, wherein at least one other of said
second plurality of reflectors is only partially reflective for
said at least one select frequency that said at least one of said
second plurality of reflectors is highly reflective.
22. The device of claim 1, further comprising a highly reflective
coating on said first facet.
23. The device of claim 1, further comprising an anti-reflective
coating on said second facet.
24. The device of claim 1, wherein a transition from said expanding
tapered portion to said contracting tapered portion is gradual.
Description
FIELD OF INVENTION
The present invention relates generally to optical devices, and
particularly to superluminescent diodes (SLD's) and lasers.
BACKGROUND OF INVENTION
There is currently a need for high power SLD's suitable for use as
optical amplifiers. It is an object of the present invention to
address this need. There is further a need for compact and reliable
projections systems. It is another object of the present invention
to address this need as well. The invention also enables one to
provide and enable external cavity lasers.
SUMMARY OF INVENTION
An optical device including: first and second facets; an at least
partially bent waveguide formed on a substrate and including a
portion perpendicular to the first facet; and, a light
amplification region coupled to the bent waveguide, the light
amplification region including an expanding tapered portion and a
contracting tapered portion wherein the contracting tapered portion
approaches the second facet.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 illustrates a double-pass bent or arcuate waveguide
superluminescent diode (SLD) utilized according to an aspect of the
present invention;
FIG. 2 illustrates a mode propagating through the waveguide of FIG.
1;
FIG. 3 illustrates a type of waveguide utilized according to yet
another aspect of the present invention;
FIG. 4 illustrates another type of waveguide utilized according to
yet another aspect of the present invention;
FIG. 5 illustrates a up-conversion fiber laser according to another
aspect of the present invention;
FIG. 6 illustrates an up-conversion laser utilizing a PPLN crystal
according to another aspect of the present invention;
FIG. 7 illustrates a display system according to an aspect of the
present invention; and,
FIG. 8 illustrates modal reflectivity of an angled stripe SLD at
1550 nm wavelength for several lateral index steps.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the figures, like references there-throughout
designate like elements of the invention. A double-pass or bent
waveguide superluminescent diode (SLD) 10 utilized according to one
aspect of the present invention is shown in FIG. 1. The SLD 10
includes a ridge waveguide portion 20, having an effective index of
refraction n.sub.e, along a substrate 30 having an index of
refraction n.sub.c. The effective index is obtained from the active
layer's bulk index n.sub.f by solving Maxwell's equation with the
waveguide's boundary conditions. The SLD 10 includes a first facet
40 having a coating of a prescribed reflection, which may be a
highly reflective coating or an anti-reflective (AR) coating and a
second facet 50 having an (AR) coating. The lateral index step for
the ridge waveguide 20 has a refractive index difference
.DELTA.n.ltoreq.0.01 according to an aspect of the present
invention. The ridge waveguide 20 width (w) is such as to maintain
a single transverse-mode, typically about 3 .mu.m for operating
wavelength in the 1000 nm range. Referring now also to FIG. 8, it
shows modal reflectivity of an angled stripe SLD at 1550 nm
wavelength for several lateral index steps. It should be noted that
selecting a tilt angle of .theta..apprxeq.6.degree. (FIG. 1) and a
waveguide 20 width (w) (FIG. 1) of about 3 microns, the mode
reflectivity is advantageously relatively constant for a .DELTA.n
ranging from approximately 0.003 to approximately 0.01. This
advantageously increases manufacturing tolerances for the waveguide
20.
Referring again to FIG. 1, the radius of curvature is given by
##EQU1##
where ##EQU2##
where n.sub.e is the effective index and X is the wavelength in
free space, and ##EQU3##
This expression can be simplified to ##EQU4##
where .DELTA.n=n.sub.e -n.sub.c. The length s of the bent region is
given by s=r.phi., where .phi. is expressed in radians. For a ridge
waveguide structure with an angle of 6.degree. (.about.0.1 radian),
the length of the bend region is 0.1r, and a robust angle design
with negligible bend loss is one for which r=10 mm and s=1 mm. For
a chip of length 1 mm, the whole bent waveguide would simply be a
circular arc, and the bend loss would be of the order of 1% or
less.
This configuration was also described in U.S. Pat. No. 6,430,207,
issued on Aug. 6, 2002, and entitled "MULTPLE-WAVELENGTH
MODE-LOCKED LASER", the entire disclosure of which is herein
incorporated by reference. Referring now also to FIG. 2, there is
shown mode propagation through the SLD 10.
A "diamond-like" shaped SLD is taught in U.S. Pat. No. 6,417,524,
issued on Jul. 9, 2002, and entitled "LIGHT EMITTING SEMICONDUCTOR
DEVICE", also hereby incorporated by reference herein. The
"diamond-like" SLD structure thereof is capable of high power
operation. This is possible because the walls of the waveguide are
non-parallel, so it does not support high order waveguide modes.
This allows its fabrication in a volume that is much larger than a
conventional narrow stripe SLD, and hence gives it the capability
for emitting high power in a single mode.
According to the present invention, the bent SLD configuration of
FIG. 1 and the aforementioned diamond-like structure are used to
produce a high-power single-mode SLD 300 as is shown in FIG. 3, for
example. The diamond structure 310, or alternatively a single taper
active structure, is placed at an angle .alpha., typically 5 to
10.degree. with respect to the front facet, and connected to a
section 312 of narrow single-mode waveguide by means of a bent or
arcuate waveguide 320 that is designed analogously to the SLD shown
in FIG. 1. According to a preferred form of the present invention,
the transition from the narrow waveguide portion to the bent
waveguide portion should be gradual and not abrupt. Likewise, it is
preferred that the transition from expanding to contracting
tapering in the diamond-like region be gradual and not abrupt or
point-like. In this configuration 300, the narrow single-mode
waveguide 312 is perpendicular to the back facet 340, so that light
reflected from the back facet 340 is re-injected into the diamond
section or taper 310. The radius of curvature (r) is chosen as
prescribed earlier in order to prevent undesirable radiation and
hence loss from the curved or bent portion 320 of the waveguide.
The width of the narrow waveguide portion 312 near the back facet
340 is such that it preferably propagates only the lowest order
mode. This width (w) is preferably about 3 microns for typical
ridge laser structures. This further serves to stabilize the
propagated mode in the tapers of the portion 310 and further ensure
single mode operation.
According to an aspect of the present invention, the back facet 340
is coated with an interference filter that provides high
reflectance, for example >95%. In some other aspects, such as
some designs of external cavity lasers, it can also be
anti-reflection coated. The front facet 350 is coated with an
anti-reflection coating to increase output power according to
another aspect of the invention. The waveguide layer structure can
take the form of any typical laser diode structure including an
active emission layer sandwiched between p and n cladding layers
deposited epitaxially on a semiconductor substrate (GaAs for
wavelength below 1,100 nm, or InP for wavelengths 1,300 nm to 2,000
nm, for example). Therefore, the waveguide structure can be
fabricated using a conventional process of photolithography,
etching, metallization, and facet coating. Upon application of an
electric current to the device, light is created by spontaneous
emission, and a small component of it propagates along the
waveguide guide where it undergoes gain by stimulated emission and
is output as Amplified Spontaneous Emission (ASE).
It should be understood that in a single-pass SLD device, the
output light is the guided ASE component emanating from the back
end of the structure and propagating with exponential gain toward
the front end or output facet. According to another aspect of the
present invention though, the output of the device 350 also
includes light emanating from the front end 350, propagating toward
the back facet 340, being reflected from the back facet 340 and
emerging from the front facet 350 after two passes through the
structure. As a result, the maximum output power in the double-pass
structure 300 is advantageously proportional to the square of the
gain of the device, whereas in the single-pass device it is only
proportional to the gain. Thus, the double-pass SLD 300 is
advantageously a more efficient gain medium than a single-pass
device.
Still referring to FIG. 3, the double-pass device 300 thereof also
offers advantages in its use as a gain medium in external cavity
lasers (ECL). In order to make a laser from the device, in the case
where the back facet has a high-reflect coating, all that is needed
is to provide a feedback partial reflection from the output facet
350, since high reflection is already provided at the back facet
340.
The laser can be made tunable by using a frequency-selective front
feedback, such as provided by a grating. The gain-bandwidth of the
SLD at 1550 nm is about 100 nm. This value would also be the tuning
range of the laser. In this configuration, the wavelength tuning
occurs at the output end. An alternate configuration of the ECL is
one in which both front and back facets are anti-reflection coated
and in which the external feedback element provides maximum
reflection. In this case, the output is taken from the non-angled
facet, separating the tuning function from the output function.
One convenient application of the configuration with the
high-reflect back coating is in the generation of high power
up-conversion light. As will be discussed, using such a
configuration one can readily generate high power blue (.about.460
nm) or green (.about.520 nm) visible light from a gain medium of
the type described herein by emitting radiation in the 910 to 930
nm range or 1020-1040 nm range, respectively.
Still referring to FIG. 3, according to another aspect of the
present invention the device 300 can be formed so as to have a
length of 2300 .mu.m and height of 600 .mu.m. The angle of the
diamond structure is preferably about 6.degree., while the angle
.theta..sub.1 defining the angle of the upper taper of the section
310 with respect to the output facet 350 is about 5.3.degree., and
the angle .theta..sub.2 defining the angle of the lower taper of
the section 310 with respect to the output facet 350 is about
6.7.degree.. The tilt angle is between 5 to 7.degree. for a weakly
guided angled stripe SLD at 1550 nm. The length of the tapered
portion 310 is preferably about 2150 .mu.m, with
L.sub.2.apprxeq.L.sub.4.apprxeq.975 .mu.m. The taper angle
.theta..sub.3 is preferably about 2.degree. or less, in order to
maintain adiabatic condition, a condition in which very little
light is converted to high-order waveguide modes. The transition of
the region 310 from tapering in an expanding manner to a
contracting manner preferably happens in a length L.sub.3 which may
be about 200 .mu.m. The length of the waveguide portion 312 is
about 125 .mu.m, which makes L.sub.5 approximately 2300 .mu.m. In
any event, L.sub.1 is approximately 125 .mu.m, L.sub.2 is 975
.mu.m, L.sub.3 is 200 .mu.m, L.sub.4 is 975 .mu.m and L.sub.5 is 25
.mu.m. Thus, the total L is about 2300 .mu.m. Basically, the length
of the arcuate or curved part 310 is selected to minimize radiation
at 1555 nm.
Referring now also to FIG. 4, according to another aspect of the
present invention, the single-mode waveguide section 412 coupled to
a diamond or taper 410 can also be applied without bending the
waveguide of the structure 400 to make a high-power single-mode
laser. In this case, both the front 450 and the back facets 440 are
perpendicular to the waveguide structure 430. Again, the back facet
440 is coated with a high-reflect filter and the front facet 450 is
coated with an anti-reflection layer to provide reflection on the
order of a few percent. Such a structure can lase without external
feedback, due to reflection provided by the back facet 400 and
front facet 450. However, it should be recognized that the
structure 400 is less suitable for external cavity lasers than the
angled structure shown in FIG. 3. For external cavity operation,
front facet reflection is preferably <0.001%. This is more
easily achieved when the waveguide 430 is not perpendicular to the
front facet 450. For example, the angled structure 300 of FIG. 3
which exhibits a 6.degree. facet angle can provide reflection on
the order of 0.001% to 0.0001%.
Still referring to FIG. 4, the portion 412 is preferably about 100
to 150 .mu.m in length while the portion 410 is preferably about
2150 to 2200 .mu.m in length. The portion 412 preferably has a
width (w) of about 3 .mu.m, while the region 410 has a maximum
width (w.sub.1) of about 38 .mu.m and the width (w.sub.2) of the
waveguide at the output facet 450 is about 10 .mu.m. The angle
.beta..sub.1 at which the taper of the region 410 expands is
preferably about 1.degree. while the angle .beta..sub.2 at which
the taper of the region 410 contracts is about 0.7.degree..
In U.S. Pat. No. 6,363,088, issued on Mar. 26, 2002, and entitled
"ALL SOLID STATE HIGH POWER BROADBAND VISIBLE LIGHT SOURCE", also
herein incorporated by reference, there is described an
up-conversion laser including a high-power diamond infrared gain
medium and a length of rare-earth-doped fluoride fiber inside a
double cavity for high conversion efficiency. A fiber laser system
500 is shown in FIG. 5.
Still referring to FIG. 5, this laser system 500 is a dual-cavity
laser consisting of an infrared (IR) laser 510 of a type discussed
hereto and that is pumped by an electric current, and a visible
laser 520 that is pumped by the IR laser 510. The IR laser 510
includes a diamond-like SLD 512, a high-reflect mirror 511 on the
left side of the diamond-like region 512, and another high-reflect
mirror 513 on the far right in FIG. 5, on an opposite side of the
visible laser 520 from the IR laser 510. IR light passes through
the fiber 521 of the visible laser 520 and is partially absorbed by
the rare-earth ions thereof to produce fluorescence at a
corresponding wavelength in the visible spectrum. The visible laser
520 includes the up-conversion fiber 521, a high-reflect mirror 522
for the visible on the left side of the fiber 521, and a
partial-reflect mirror 523 on the right side of the fiber 521. The
mirrors 522, 523 are preferably interference mirrors and provide
reflection only at the intended wavelength. Thus, the visible
reflectors 522, 523 are transparent to the IR light, and
vice-versa.
Referring now to FIG. 6, according to another aspect of the present
invention the up-conversion fiber 521 and external reflectors 522,
523 are replaced by a crystal 610 such as a periodically-poled
lithium niobate (PPLN) crystal 610, which includes a high-reflect
visible light reflector 611 in the back facet 614 thereof, a
high-reflect IR reflector 612 and a partial-reflect visible
reflector 613. The configuration 600 of FIG. 6 also includes
high-reflect reflector 621 on the back facet of the bent
diamond-like SLD 620. Alternatively, a single tapered SLD could be
used. FIG. 6 show an embodiment of the double-pass SLD dual cavity
laser with the PPLN configured as a frequency converter. According
to an aspect of the present invention, frequency conversion is
obtained by frequency doubling due to the non-linearity of the PPLN
material characteristics. For example, if the IR pump wavelength is
in the 910-930 nm range, the process of frequency doubling
generates blue light at 455 to 465 nm. Similarly, if the IR pump
wavelength is at 1020 to 1040 nm, then the laser will generate a
510 to 520 nm green light.
A dual-cavity laser system according to the present invention is
well suited for generating primary light sources for color
projection systems. It is an advantageously compact, being a few
centimeters in dimensions for example, point source which exhibits
a low beam divergence, and is suitable for high efficiency
projection systems, with high saturation colors and a long
lifetime. According to another aspect of the present invention,
such systems can produce visible output power in the 1 to 10 watt
range.
Referring now to FIG. 7, according to yet another aspect of the
present invention the light sources for a projection system 799
include one source of the type described in FIG. 5 or 6 with the
appropriate IR wavelengths for the generation of blue and green,
respectively 720, 730. While, according to another aspect of the
present invention, a red primary beam can be generated directly
from the semiconductor at 630 to 650 m using the structure 710
shown in FIG. 4, and discussed above, for example. Outputs from the
sources 710, 720, 730 are supplied to a polarization cube 740 which
feeds a projector 750, for example.
Although the invention has been described and pictured in a
preferred form with a certain degree of particularity, it is
understood that the present disclosure of the preferred form, has
been made only by way of example, and that numerous changes in the
details of construction and combination and arrangement of parts
maybe made without departing from the spirit and scope of the
invention as hereinafter claimed. It is intended that the patent
shall cover by suitable expression in the appended claims, whatever
features of patentable novelty exist in the invention
disclosed.
* * * * *